1. Field of the Invention
The present invention relates to the preparation of metallic oxides having a bridged lamellar structure and to the metallic oxides thus produced.
This invention more especially relates to the bridging of metallic oxides having a lamellar structure to produce metallic oxides having a high specific surface.
2. Description of the Prior Art
Considerable research has to date been carried out to prepare materials having a high specific surface which are particularly useful in adsorption, absorption or catalysis. The zeolites are the materials currently most typically used for these applications. Nonetheless, this art is actively seeking substitutes for the zeolites. Indeed, the zeolites have certain disadvantages, in particular the limitation on the size of the channels extending therethrough.
Among such possible substitutes, compounds having a bridged lamellar structure are considered especially interesting. Thus, numerous bridged clays comprising a variety of bridging structures are already known to this art.
By "bridging" is intended the insertion of ionic or nonionic chemical species between the lamella of the lamellar structure of a product, causing a swelling of the product by the spreading apart of the different planes, followed by the physical or chemical joining of said species with the lamella of the lamellar structure.
In the case in which the species inserted are not bonded, the phenomenon is reversible and is designated intercalation.
The intercalation and bridging of different metallic oxides having a lamellar structure are also known to this art. Thus, vanadium pentoxide gels have a lamellar structure and are characterized by colloidal particles in the form of strips which are capable of being stacked upon each other and which serve as the host for numerous chemical compounds, such as organic solvents, long chain cations or metallocenes, as described in the article of J. Livage et al.
The number of species that may be inserted, and in particular the cations, depend on the charge and the numerous sites present on each lamella or plane of the lamellar structure. In the case of clays, the charge is due to substitution of cations of silicon and/or aluminum for cations having a lesser charge. In vanadium pentoxide gels, the charge may be obtained by the reduction of vanadium cations. The intercalation then involves a transfer of electrons, combined with the insertion of the species to be intercalated into the lattice. However, this insertion may modify the structure or destroy the lattice, commencing at a certain degree of reduction.
To date it has not been possible to insert highly charged, large ionic species, such as of [Al.sub.13 O.sub.4 (OH).sub.24 ].sup.7+ type, into metallic oxides of vanadium pentoxide type, to effect particularly desirable catalytic properties.